The invention relates to electromagnetic energy, and more particularly, to electromagnetic exposure of planar materials.
Microwaves can be used to heat paper and other planar materials. It is well known in the art to use a slotted waveguide that has a serpentine path in order to maximize the exposure area of the material passed through the waveguide. See, for example, U.S. Pat. No. 5,169,571; U.S. Pat. No. 4,446,348; and U.S. Pat. No. 3,765,425. Conventional waveguides have four particular drawbacks. First, the microwave signal attenuates as it moves away from its source. This attenuation versus propagation distance increases when lossy planar materials are introduced into the waveguide. As a result, a material fed into the waveguide through a slot is heated more at one end of a segment (closer to a source) than at the other end (farther from a source). Prior art structures have not made use of the slot's orientation as a means for addressing this problem. In a traditional slotted waveguide, there is a field peak midway between two conducting surfaces. In the prior art, the slot is at this midway point. See, for example, U.S. Pat. No. 3,471,672, U.S. Pat. No. 3,765,425, and U.S. Pat. No. 5,169,571.
A second problem relates to the distribution of the microwave energy. Because the magnitude of the electric field in a microwave signal has peaks and valleys due to forward and reverse propagation in the waveguide, planar materials fed through a slotted waveguide tend to experience hot spots. U.S. Pat. No. 3,765,425 (hereinafter, "the '425 patent") addresses this problem through the use of two disconnected waveguides that are interspersed with each other. At least one waveguide is equipped with a phase shifter to ensure that the hot spots in one waveguide occur at locations different than in the other waveguide. One disadvantage to this approach (aside from the expense of a phase shifter) is that sections of separate waveguide must lay on top of one another in order for planar materials to experience alternating hot spots as they pass through the entire structure. Furthermore, each distinct variation in phase requires an additional serpentine waveguide and an additional microwave source.
Another attempt to smooth out the effect of "hot spots" is disclosed in U.S. Pat. No. 5,536,921 (hereinafter, "the '921 patent"). Like the '425 patent, the '921 patent also depends on separate and distinct sections of waveguide. However, instead of using one or more phase shifters, the '921 patent offsets its separated sections of waveguide by exactly a 1/4 of a wavelength. One disadvantage of this approach is that it requires more than one phase-controlled path. The '921 patent requires even more paths than the '425 patent. According to the '921 disclosure, each waveguide section for exposing materials is a separate wave path. Each such section requires its own point for launching the wave and its own termination point. Each launching point inevitably has losses due to signal reflection.
In addition, the approach disclosed in the '921 patent does not allow for easy adjustment to adapt to a variety of materials. It will be appreciated by those skilled in the art that the actual length of a 1/4 wavelength is dependent on the material introduced into the waveguide. Therefore, the '921 patent teaches a device that must be built for a specific material. If the constructed device was used for a material with a different .epsilon..sub.r, the 1/4 offset and its benefits would be reduced or completely eliminated. For example, if the structure disclosed in the '921 patent were used on a material whose .epsilon..sub.r was different by a factor of 4 from the .epsilon..sub.r of the material for which the structure was designed, then the material would be exposed to similarly placed (rather than offsetting) hot spots. It will also be appreciated by those skilled in the art that to further smooth out the effect of hot-spots, it may be advantageous to space hot spots by less than a 1/4 of a wavelength. Applicants co-pending application #08/848,244, now U.S. Pat. No. 5,958,275, which is herein fully incorporated by reference, discloses an adjustable structure that can be used to heat a variety of materials.
Another attempt to smooth out the effect of "hot spots" is disclosed in U.S. Pat. No. 4,234,775 (hereinafter, "the '775 patent"). The '775 patent, like the '425 and '921 patents, uses a single frequency to try and uniformly heat a material. However, the '775 patent uses a tuning plunger, a rotating head, and a dielectric material to "substantially disrupt" the standing wave. One problem with this approach is that it is difficult to predict how the peaks and valleys will realign when the standing wave is disrupted. While purposely disrupting the standing wave shifts the peaks and valleys, it does not guarantee that the material is more evenly heated. It is important to note that because the '775 patent disrupts the wave, it is advantageous to place the rotating head at the end of the waveguide.
It will be appreciated by those skilled in the art that the distance between consecutive peaks depends on the frequency of the wave. If the frequency is increased, the distance between consecutive peaks decreases. If the frequency is decreased, the distance increases. Only recently, researchers have begun to realize that it is possible to vary the frequency of a wave in a multimode cavity to generate more uniform heating. See, for example, U.S. Pat. No. 5,879,756; U.S. Pat. No. 5,804,801; and U.S. Pat. No. 5,798,395. While researchers have experimented with using a variable frequency to generate a plurality of modes, Applicants are not aware of any references that teach how to use a variable frequency in a slotted waveguide to more uniformly heat a planar material.
A third problem with traditional waveguides for electromagnetic exposure relates to the field gradient between top and bottom conducting surfaces. This gradient does not pose a problem if the planar material is of an insignificant thickness. However, if the planar material does have an appreciable thickness, this gradient can lead to nonuniform heating. One way to overcome this problem is disclosed in Applicants' co-pending applications #08/813,061 and #08/848,244, now U.S. Pat. No. 5,998,774 and U.S. Pat. 5,958,275, respectively. These co-pending applications, which are herein fully incorporated by reference, disclose the advantages of a dielectric slab-loaded structure that elongates the peak field region in a single mode cavity. However, slab-loaded structures have not yet been adapted for exposure of planar materials.
A fourth problem relates to leakage of microwaves through the slot of a slotted waveguide. Energy leakage and radiation is a general problem for any microwave structure. The problem of radiation through open access points is magnified when the material being passed through the structure has any electrical conductivity. Such conductive substances (for example, any ionized moisture in paper that is passed through a chamber for drying) can, when passed through a microwave exposure structure, act as an antenna and carry microwaves outside the structure's cavity.
There are several different ways to address the problem of leakage through the slots of a slotted waveguide. One approach is to enclose the entire slotted waveguide in a reflective casing. See, for example, U.S. Pat. No. 5,169,571. This approach has obvious drawbacks. If the reflective casing does not itself have access points that remain open during the delivery of a microwave field, then the feed-through process must be fully automated and must exist inside the outer casing. On the other hand, if the reflective casing does have access points that remain open during the delivery of a microwave field--as does the structure disclosed in U.S. Pat. No. 5,169,571--then there is still a problem of leakage through those access points.
A second approach is the use of a reflective curtain or flap draped over the slot. U.S. Pat. No. 5,470,423 discloses such an approach. That patent discloses the use of conductive flaps or "fingers" (see "fingers 110" in FIG. 1). Although such a conductive curtain may reduce leakage, it may also tend to obstruct smooth passage of any material that is fed through the slot. Any contact between such a curtain and any material tends to disrupt the surface tension of the material. Moreover, damaging arcing may occur between the curtain and the material. Furthermore, a reflective curtain does nothing to reduce the problem of an electrically conductive material's tendency to act as an antenna--alone or in combination with a waveguide's exterior conducting surface--and thus radiate energy through the slot.
A third approach is the use of a choke flange. Chokes that prevent the escape of electromagnetic energy from the cracks between two imperfectly contacting surfaces are well known in the art. Particularly well known are chokes designed for microwave oven doors and waveguide couplers. See, for example, U.S. Reissue Pat. No. 32,664 (1988); U.S. Pat. No. 3,843,861. What has not been fully explored in the art is the use of the choke flange concept to reduce leakage through arbitrarily shaped access points that remain open during delivery of a microwave field. U.S. Pat. No. 4,999,469 (hereinafter, "the '469 patent") discloses a choke flange that can be used with a slotted waveguide. The '469 patent discloses a choke flange that has a vertical section that precedes a horizontal section. Although choke flanges have been used to reduce leakage through a continuously open opening, the present invention and co-pending applications #08/813,061 and #08/848,244, incorporated herein by reference, now U.S. Pat. No. 5,998,774 and U.S. Pat. No. 5,958,275, respectively, describe how the choke flange concept can be improved to decrease the amount of leakage. One problem with the choke flange in the '469 patent and some of the choke flanges disclosed in our earlier applications is that the choke flange can act as an antenna radiating the energy that travels along it.